The present disclosure relates to geotechnical structures and processes for forming the same. Among other advantages, the geotechnical structures are cost-effective and improve on deficiencies associated with the materials normally used in prior art structures.
A cellular confinement system (CCS) is an array of containment cells resembling a “honeycomb” structure that is filled with granular infill, which can be cohesionless soil, sand, gravel, ballast, crushed stone, or any other type of granular aggregate. Also known as geocells, CCSs are mainly used in civil engineering applications that require moderate mechanical strength and stiffness, such as slope protection (to prevent erosion) or providing lateral support for slopes, as well as for providing limited vertical load support, usually for temporary unpaved roads.
CCSs differ from other geosynthetics such as geogrids or geofabrics in that geogrids/geofabrics are flat (i.e., two-dimensional, with very small height in relation to length and width) and used as planar reinforcement. Geogrids/geofabrics provide confinement only for very limited vertical distances (usually 1-2 times the average size of the granular material) and are limited to granular materials having an average size of greater than about 20 mm. This limits the use of such two-dimensional geosynthetics to relatively expensive granular materials (ballast, crushed stone and gravel) because two-dimensional geosynthetics provide little confinement or reinforcement to finely-sized granular materials, such as sand, crushed concrete and quarry screenings.
In contrast to the above, CCSs are three-dimensional structures that provide confinement in all directions (i.e. along the entire cross-section of each cell). Moreover, the multi-cell geometry provides passive resistance that increases the bearing capacity. Unlike two-dimensional geosynthetics, a geocell provides confinement and reinforcement to granular materials having an average particle size less than about 20 mm, and in some cases materials having an average particle size of about 10 mm or less. However, current geocells are made of polyethylene (usually medium or high density polyethylene, referred as MDPE and HDPE).
As used herein, the term “geotechnical structure” refers to the combination of (1) a geosynthetic article, such as geogrids, geofabrics, and geocells, including combinations thereof; with (2) an infill granular material such as soil, crushed rock, sand, crushed stone, crushed concrete, and earth materials. Geotechnical structures generally have increased load bearing capacity, stability, and erosion resistance compared to the infill material itself.
In particular, geocells contribute to the strength of the surrounding materials and materials contained within the cells in several ways. First, the lateral stress exerted by the cell walls on the infill contained therein increases when a compressive stress is applied to the surface of the geocell. The increase in the lateral, confining stress can be as large as the increase in the applied compressive stress. Because the strength of the infill material depends on the lateral stress, an increase in the lateral stress increases the strength of the infill material. In fact, using a stiff wall to confine the infill would create a situation where any increase in compressive stress will resemble a state of hydrostatic stress increase (i.e., the stress increases equally in all directions). This results in only a small shear stress in the confined infill. As a result, the confined infill exhibits a greater lateral strength for a given depth, compared to unconfined fill.
This principle can be illustrated by reviewing the characteristics of soil at various depths. Heaped as a pile on a surface, soil has zero confinement and thus zero strength when a compressive stress is applied (i.e., a mound of soil flattens when pressed down upon). However, when confined, such as when the soil is in the ground, trying to drive a stake into the ground gets more difficult the deeper one tries to drive it, i.e. the strength of the soil increases. This is because the deeper soil is confined, and thus cannot move laterally to relieve the stress placed upon it.
Typical infill materials for geotechnical structures are naturally available materials or low cost materials. Such infill materials include recycled asphalt concrete (RAP), naturally abundant sand (such as river sand), crushed concrete, crushed bricks, or recycled plastics or rubber. Some problems that result from using these materials include the high tendency of crushed concrete and bricks to absorb water through capillary mechanisms; the tendency of RAP to creep under heavy loads, a tendency that become worse as temperature increases; poor resistance of sand to water and wind erosion; and the lack of a granular skeleton or cohesion for organic aggregates like recycled plastics. Because of the relatively high content of fines and the lack of cohesion, these materials are not generally used in structural applications intended for long periods of time.
It would be desirable to develop a geotechnical structure that utilizes low-cost granular materials for structural applications such as roads, parking lots, or railways, and improves the drawbacks associated with the materials normally used in prior art structures.